Changing the coolant on my '03 Camry. The tech at the local Toyota dealer said that using DEX-Cool would be fine. Do these systems really need to be flushed thoroughly before putting in fresh coolant? If DEX-Cool mixes with the old Toyota coolant will it hurt anything?

Also, my truck has an aluminum radiator with the old green coolant...I should be running DEX-Cool with an aluminum radiator should't I?

Thanks for any input!